To measure the magnetic field strength in the solar corona , we examined 10 fast ( \geq 1000 km s ^ { -1 } ) limb CMEs which show clear shock structures in SOHO/LASCO images .
By applying piston-shock relationship to the observed CME ’ s standoff distance and electron density compression ratio , we estimated the Mach number , Alfven speed , and magnetic field strength in the height range 3 to 15 solar radii ( R _ { s } ) .
Main results from this study are : ( 1 ) the standoff distance observed in solar corona is consistent with those from a magnetohydrodynamic ( MHD ) model and near-Earth observations ; ( 2 ) the Mach number as a shock strength is in the range 1.49 to 3.43 from the standoff distance ratio , but when we use the density compression ratio , the Mach number is in the range 1.47 to 1.90 , implying that the measured density compression ratio is likely to be underestimated due to observational limits ; ( 3 ) the Alfven speed ranges from 259 to 982 km s ^ { -1 } and the magnetic field strength is in the range 6 to 105 mG when the standoff distance is used ; ( 4 ) if we multiply the density compression ratio by a factor of 2 , the Alfven speeds and the magnetic field strengths are consistent in both methods ; ( 5 ) the magnetic field strengths derived from the shock parameters are similar to those of empirical models and previous estimates .